1. Field of the Invention
The present invention relates to a silica glass crucible used for pulling up single crystal silicon used for a semiconductor from melted silicon.
2. Discussion of the Prior Art
In recent years, a silica glass crucible having a large open diameter has been used in accordance with an enlargement of a silicon wafer, and a thermal load on the silica glass crucible becomes to be increased since a melting amount is increased and a pulling up time is long, thus there is a problem that the silica glass crucible is sunk during the pulling up. Then, the silica glass crucible being reinforced by crystallizing an inner or outer surface of the crucible has been known for preventing the sinking.
For example, in Japanese Patent Laid Open No. Hei09-110590, it was described that the crucible can be reinforced by coating an agent for accelerating the crystallization on the outer surface of the crucible and crystallizing the crucible during the pulling up. However, the agent for accelerating the crystallization is an impurity for the single crystal silicon and may have a bad influence on an electric characteristic of the produced wafer. Further, in Japanese Patent Laid Open No. Hei10-203893, it was described that an oxyhydrogen flame is sprayed on the outer surface of the crucible to form a crystallized glass layer on the outer surface of the crucible. When the silica glass is heated at more than a softening point under an oxygen atmosphere, cristobalite is deposited during a cooling process. However, since a thermal expansion coefficient of the silica glass is much different from that of cristobalite, the cristobalite layer formed by said method is easily released, thus this method is not suitable for utilization.
On the other hand, in Japanese Patent Laid Open No. 2001-328831, the silica glass crucible was described, wherein the whole inner surface of the crucible is polished by a sand-blast or the like, and the polished surface is further heated and treated with the oxyhydrogen flame to be smoothed. It was pointed out that said silica glass crucible had an advantage that a ratio of single crystallization could be increased since bubbles were not contained in the inner surface and the purity was high. However, the problem of the sinking of the crucible is not solved.